Mother Teresa famously said: “If I look at the mass, I will never act. If I look at the one, I will.” There are worse people to turn to for lessons in human charity, and here Calcutta’s celebrated missionary also showed an astute grasp of cognitive psychology—and its paradoxes. Our compassion and generosity should grow as the number of poor and suffering multiplies, but the opposite seems to occur. Some numbers are just too big and abstract to grasp, so they lose their power.

Modern charities might take a lesson from this quirk of human thinking. And indeed, psychological scientist Christopher Hsee of the University of Chicago’s Booth School of Business has been studying Mother Teresa’s wisdom, called “scope insensitivity” in the less poetic jargon of the field. This simply means that, when asked to give money to help some number of needy people, say 100—we ignore the number 100. We can’t verify that number anyway, so instead we substitute easy-to-read cues, like our feelings for a single needy person. We focus on a prototype of all needy people. Hsee has been using this cognitive principle to come up with practical advice for those in the philanthropy business.

Here’s how Hsee (working with colleagues at the University of Miami) has been studying this concept in the lab. He figured he could exploit the human tendency to focus on the unit of one. That is, if people do respond to the prototypical needy person more powerfully than to 100 needy people, then they should be as generous toward this single needy individual as they would be toward a group of 100. They should donate as much to one as they would to all 100—or close to it—since that one represents the collective need. The donation they choose, he reasoned further, would then become the designated donation—the baseline—for any one needy person. Then, if subsequently asked how much they are willing to give to 100 needy people, they would give a lot more to the group–perhaps not 100 times 1, but substantially more, depending on their means.

That’s at least the theory, and here’s an experiment Hsee ran to test it. He asked volunteers to imagine that an elementary school principal asked them to donate money so that 20 low-income kindergarteners could get Christmas gifts. The request included a portrait of one of the needy children, a little girl. Some of the volunteers were told: “Please think about all of these 20 children. How much are you willing to donate to help these 20 children?” These volunteers served as controls. Others were also asked this but—before this request—they were told to focus on one child: “How much would you donate to help this one child?” So the only difference was that some volunteers first came up with—and wrote down—a hypothetical amount they would be willing to give to a single needy kindergartener.

These volunteers were more generous in the end. In fact, they contributed almost twice as much as the controls did. And it was clearly because of the focus on one: As Hsee had anticipated, these volunteers gave about as much to one child—the prototype—as the controls did to the entire 20. In other words, the volunteers didn’t respond to the number 20 at all—only to the number 1. And they used this prototypical donation to calculate their total giving.

Hsee is careful to distinguish his results from a phenomenon well known to fundraisers—the “identifiable victim effect.” This is the human tendency to be more helpful to actual individuals—with a name and face—than to anonymous victims. It’s why solicitations often include a photograph of an adorable, but clearly needy, child. In this study, since all of the volunteers saw the same child’s photo, the uptick in generosity was clearly not the result of the photo and its emotional manipulation.

Lab studies have their limits, of course, and Hsee wanted to double-check these results in a more realistic setting. So in a second experiment, he used basically the same methods in an actual fundraiser at a mid-sized company in China. This company, with about 800 workers, runs annual fundraising events for the needy, and this one aimed to help 40 young schoolchildren who were victims of the devastating 2008 earthquake in Sichuan province. The company sent an e-mail to all employees encouraging them to make a private donation at a designated website. But there were in fact two versions of the website, which replicated the first experiment: Some were asked, before making their actual donation, to indicate how much, hypothetically, they would give to just one of the needy children. The controls were simply asked to make a donation for the entire 40 children.

The results were clear, and much like the earlier findings. As described in a forthcoming article in the journal Psychological Science, those who were first prompted to focus on a single earthquake victim gave much more in the end than did the controls. The cognitive potency of the number 1 significantly shaped real-life acts of charity.

The US is slowly grinding its way out of the recession, and charitable giving is showing some signs of emerging from the doldrums. But need is greater than ever, and philanthropic fundraising is a costly endeavor. Hsee’s work may offer a simple, no-cost strategy to get donors to think differently about their capacity for generosity.